The present invention relates to a printer which is connected to a computer network, for example, so that the printer is shared by a plurality of users, and a printing method thereby.
In general, an electrophotographic copying machine or a printer is required to drive a photosensitive drum for a predetermined time in advance upon starting the printing operation so as to stably drive the photosensitive drum and stabilize the voltage of the photosensitive drum. Further, after completion of the printing operation, the photosensitive drum is required to be driven for a predetermined period so as to perform the post printing operation such as the removal of the toner remained after the transfer process of the last sheet to be printed and the removal of the history of the formed image.
The life time of the photosensitive drum is known to be determined substantially by the abrasion amount of the photosensitive layer. The abrasion amount is substantially proportional to the rotation time period of the photosensitive drum, so that the higher the ratio of the actual printing time period with respect to the rotation time period of the photosensitive drum (that is, using efficiency of the photosensitive drum) is, the larger the number of sheets being printed within the life time of the photosensitive drum becomes. That is, it has been known that the more effectively the photosensitive drum is used, the lower the printing cost per one sheet becomes.
FIG. 2 is a graph showing a relation between the printed sheet number and the .rotation number of the photosensitive drum per one printed sheet. The relation shown in FIG. 2 is calculated under a condition that a Black & White small printer is used, a photosensitive drum with .phi.30 is used, the photosensitive drum is required to be rotated by one revolution each upon starting and completing the printing operation, a sheet of A4 size is fed to the longitudinal direction thereof, and an interval of sheets to be continuously printed is 25 mm. When the printed sheet number and the rotation number of the photosensitive drum per one sheet are briefly calculated under such a condition, the relation shown in FIG. 2 is obtained. In this respect, the rotation number of the photosensitive drum per one sheet is inversely proportional to the continuously printed sheet number. Thus, when the continuously printed sheet number is small, a ratio of the rotation number of the photosensitive drum required upon starting and completing the printing operation with respect to the total rotation number from the start to the completion of the printing operation will be large, and hence the rotation number of the photosensitive drum per one sheet will be large. In contrast, when the continuously printed sheet number becomes large, the rotation number of the photosensitive drum required upon starting and completing the printing operation will less influence to the rotation number of the photosensitive drum per one sheet. It will be understood from the graph shown in FIG. 2 that, when the continuously printed sheet number is not more than five, the rotation number of the photosensitive drum per one sheet rapidly decreases with the increase of the continuously printed sheet number. Accordingly, it will be understood that this printer is disadvantageous in view of printing cost when the continuously printed sheet number is not more than four. In order to improve such disadvantage, it is important to decrease to the utmost the rotation number of the photosensitive drum required upon starting and completing the printing operation or to increase the continuously printed sheet number. While the explanation has been made as to the photosensitive drum, it has been found that the aforesaid fact is applied to elements other than the photosensitive drum such as a cleaning blade, brush, corotron, fuser roll or the like.
A tandem type color printer of belt transfer type has been known as an example of the printer. FIG. 3 shows a diagram showing the construction of an example of the tandem type color printer. In the figure, a reference numeral 1 depicts a transfer belt and 2 to 5 depict photosensitive drums. This color printer is provided with image forming units for colors K (black), Y (yellow), M (magenta) and C (cyanogen) to be used, respectively, for example. The photosensitive drums 2 to 5 are arranged at the image forming units, respectively. The toner images formed on the photosensitive drums 2 to 5 are sequentially transferred to a sheet which is electrostatically adsorbed on the transfer belt 1 and then fused, whereby a color image is formed on the sheet.
The transfer belt 1 is formed by adhesively connecting opposite ends of a dielectric sheet so as to shape a belt-like configuration. Thus, the transfer belt includes a seam portion. Since the seam portion is not suitable for the transfer operation, it is necessary to detect the seam portion of the transfer belt and control the printer so that a sheet is not positioned on the seam portion. To this end, a mark corresponding to the seam portion is formed on the transfer belt so that the seam portion is detected by optically detecting the mark.
An example of such a color printer is configured that a process speed is 200 mm/sec, the peripheral length of the transfer belt 1 is 2000 mm, the diameter of each of the photosensitive drums 2 to 5 is 80 mm and a distance between the shafts of the adjacent photosensitive drums is 200 mm.
When the printer is started, all the photosensitive drums 2 to 5 and the transfer belt 1 are operated. Firstly, the seam portion of the transfer belt 1 is detected and the transfer belt is controlled in its timing so that the sheet is not placed on the seam portion. Then the sheet is adsorbed on the transfer belt 1 and sequentially fed to the image forming units. Toner images are formed on the respective photosensitive drums 2 to 5 and transferred on the sheet. After the transfer process, the photosensitive drums 2 to 5 and the transfer belt 1 are cleaned, whereby the printing operation is completed.
In such a printing operation, in order to obtain one printed sheet, the transfer belt 1 is moved so that the toner images formed on the four photosensitive drums 2 to 5 are sequentially transferred to a sheet. Thus, the printing cost per one sheet of such a color printer will be four times as large as that of the Black & White printer. Further, each of the photosensitive drums 2 to 5 for the respective colors are required to be rotated excessively so as to move a sheet by a distance corresponding to the respective intervals between the adjacent photosensitive drums. Further, since the transfer belt 1 is controlled in its position so that a sheet is not positioned on the seam portion, the photosensitive drums are inevitably required to be rotated excessively. In this manner, the color printer shown in FIG. 3 requires the aforesaid sequential transfer operations by the photosensitive drums 2 to 5, the excessive rotation of the photosensitive drums 2 to 5 due to the intervals thereof, the positional control of the transfer belt for avoiding the seam portion or the like, whereby the printing cost per one sheet of such a color printer will be four times or more as large as that of the Black & White printer. As a result, when the continuously printed sheet number is small, the printing cost per one sheet will remarkably increase with the decrease of the printed sheet number in the case of the color printer when compared with the Black & White printer.
In the case of continuously printing a plurality of images, since sheets can be fed with an interval of 40 mm in a case of A4 size sheet, for example, the operation time period per one sheet required for the printing operation will decrease with the increase of the continuously printed sheet number. Table 1 is a diagram used for explaining an example of the relation between the continuously printed sheet number and the respective operation time periods per one sheet. As shown in the figure, a time period required for detecting the seam portion before the printing operation and a time period required for the cleaning operation after the completion of the printing operation are set to be fixed time periods. As will be apparent from Table 1, the operation time period required for printing one sheet is inversely proportional to the continuously printed sheet number like the case of FIG. 2. As described above, since the life time of the photosensitive drum is substantially proportional to the operation time period of the image forming unit, the printing cost per one sheet can be decreased with the increase of the continuously printed sheet number.
The color printer shown in FIG. 3 has been developed in particular so as to print a large number of sheets at a high speed. In most cases, such a color printer is connected to a network or the like and shared by many users, for example. Further, a printer capable of being connected to a plurality of host devices is disclosed in JP-A-2-113324, for example. Such a printer shared by many users will be placed in general at a common place such as a central printing room in view of the using state thereof. In this case, since the printer is accessed at random by many users, the average value of the continuously printed sheet number decreases. Accordingly, such a color printer is disadvantageous in that the printing cost is high.